The problem of the
non-classical ion

A cornerstone of the classical
theory of structural chemistry since the time of
Kekulé in the 1860s is that carbon can bind at
most four other atoms (tetra-coordination).Around 1950 S.
Winstein in the USA found a short-lived carbocation
that contained penta-coordinated carbon. He named the
ion non-classical. This pioneering and controversial
discovery initiated a vigorous academic feud.
H.C.
Brown, who received the 1979 Nobel Prize in
Chemistry for his development of boron compounds into
important reagents in organic synthesis, questioned
Winstein's results.
Winstein claimed that the
norbornyl cation had a structure containing a
penta-coordinated carbon i.e. that it was a carbonium
ion.
Brown on the other hand
concluded that the norbornyl cation was a
tri-coordinated cation (carbenium ion) that rapidly
rearranged into an identical tri-coordinated ion in
which the positive charge had moved to another
carbon.
Despite very great efforts by
many leading physical organic chemists, the problem
remained unsolved until Olah's method of preparing
long-lived carbocations was applied. He was able to
study the norbornyl cation directly with NMR
spectroscopy. The ion proved to have the structure
that Winstein suggested.